EP1369711A1

EP1369711A1 - Optical functional sheet

Info

Publication number: EP1369711A1
Application number: EP02705150A
Authority: EP
Inventors: Hiromitsu Takahashi; Motoyuki Suzuki
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2001-03-16
Filing date: 2002-03-13
Publication date: 2003-12-10
Also published as: US20040047042A1; KR100859906B1; EP1369711A4; CN1463368A; KR20030004401A; WO2002075375A1; US6937399B2; US20050243428A1

Abstract

By providing the optical functional sheet of the present invention which is characterized in that, at least within the sheet, a light diffusing phase and a transparent phase extending along a direction perpendicular to the surface of the sheet are alternately arranged along the sheet surface direction, the optical functional sheet which have a smooth surface that can be surface-treated or be laminated with other functional sheet and which is provided with the light collecting effect due to the internal arrangements thereof, and which involves the internally light collecting function that is capable of collecting both of light diverging transversely and light diverging longitudinally by using a single sheet, is obtainable, thus the sheet can be useful for applications such as the backlight for the liquid crystal displays.

Description

Technical Field

The present invention relates to optical functional sheets that are employed for light fittings or preferably for the use of backlight of various displays, in particular liquid crystal displays.

Background Art

In recent years, various types of displays are employed for all sort of applications such as portable devices, personal computers, monitors or televisions. Among them, liquid crystal displays are widely used for various applications which includes miniaturized products for portable devices and which recently spreads to applications for larger products such as monitors and televisions. The liquid crystal display itself is not a luminous object, and becomes possible to display by introducing light from the backside with a backlight.
It is required for the backlight not only to simply irradiate the light, but also to make the entire image illuminating uniformly and brightly. Thus, an optical functional sheet such as a light diffusive sheet or a prism sheet is commonly added for the purpose of uniformly illuminating the backlight. More specifically, the backlight commonly comprises a light diffusive sheet that equalizes the distribution of light emission across the light guiding plate, and additionally employs the prism sheet in piles that collects light in normal direction to the sheet surface for the purpose of improving brightness.
The prism sheet is a sheet having a configuration in which a number of prisms having a generally triangular cross section are arranged, and the use of this sheet provides effectively collecting light from the backlight into the normal direction to the sheet surface, thereby improving brightness of the normal direction to the sheet surface (see, for example, US Patent No. 5,161,041).
However, since the columns of prisms located on the surface of the prism sheet is very delicate and has a conformation of having sharp apexes, it may readily scratch the surface during the production or the handling, and thus provides a drawback of deteriorating the quality of images. In addition, an additional light diffusive sheet having lower diffusivity is used in piles for the purpose of hiding the line of prism columns, thereby increasing the number of sheets used for the backlight.
In addition, further improvements on the performances and the effectiveness or further improvements on reducing the thickness and the weight thereof are required for the optical functional sheet used for these backlights, and it is useful for achieving these requirements to employ implements of functional integrations such as applying a functional layer on the surface and sticking another sheet thereon, and the like. However, the surface treatment is not available for the type of the sheet such as, typically, the prism sheet, which presents its performances derived by the surface configuration.
The present inventors have found, as a result of their various investigations carried out for the purpose of solving the problems indicated above, a sheet having a function of internally collecting light which is not derived by its surface configuration, and thus the present invention is achieved.
The object of the present invention is to provide an optical functional sheet which is capable of providing a function of collecting light derived by its internal configuration without utilizing any function of collecting light derived by its surface configuration, and which can significantly improve the brightness of normal direction to the sheet surface in the case of being used for applications such as a backlight for liquid crystals. Further, it presents a conjugated optical functional sheet having the function of collecting light as well as other functions.

Disclosure of Invention

An optical functional sheet of the present invention is characterized in that, at least within the sheet, a light diffusing phase and a transparent phase extending along a direction perpendicular to the surface of the sheet are alternately arranged along the sheet surface direction.
Further, the optical functional sheet of the present invention preferably includes the following requirements:
(a) The light diffusing phases and the transparent phases are disposed in a manner that the light diffusing phases are continuous phases which are mutually coupled in the direction toward the sheet surface, and that the transparent phases are discontinuous phases which are decoupled by the light diffusing phases;
(b) The shape of projective image of the transparent phase onto the sheet surface is selected from the group consisting of trigon, tetragon, hexagon, circle and ellipsoid;
(c) The light diffusing phase is a transparent matrix component, in which fine particulate matter having different refractive index from that of the matrix component itself is dispersed;
(d) The light diffusing phase contains a number of bubbles, and the transparent phase does not contain bubbles;
(e) The ratio of the length L of the transparent phase in the direction along the film thickness of the sheet to the length p of transparent phase in the direction along the sheet surface: (L/p), in any cross section of the sheet, is 2-10;
(f) The shapes of the cross sections of the light diffusing phases and the transparent phases are selected from the group consisting of rectangle, square, parallelogram, trapezoid, trigon, campanulate or bell shape, horn shape, ellipsoid or the mixture thereof;
(g) The ratio of the length L' of the light diffusing phase in the direction along the film thickness of the sheet to the length q of light diffusing phase in the direction along the sheet surface: (L'/q), in any cross section of the sheet, is not less than 1;
(h) The areal ratio of area of the light diffusing phase to area of the transparent phase within the surface of the sheet is 1/50 - 1/1;
(i) The film thickness is 10 - 500 µm;
(j) The sheet is a combined optical functional sheet which is formed by laminating (1) any one of the optical functional sheets described above and (2) other optical functional sheet;
(k) The other optical functional sheet (2) is a light diffusive sheet comprising a transparent matrix component in which fine particulate matter having different refractive index from that of the matrix component is dispersed; or
(l) The sheet is an optical functional sheet adopted for the backlight of the liquid crystal displays.

Brief Description of the Drawings

Fig. 1 includes schematic views showing cross sections of a preferred embodiment of the optical functional sheet according to the present invention;
Fig. 2 includes schematic perspective views showing a preferred embodiment of the optical functional sheet according to the present invention;
Fig. 3 includes schematic plan views of a preferred embodiment of the optical functional sheet according to the present invention;
Fig. 4 includes schematic plan views of another preferred embodiment of the optical functional sheet according to the present invention;
Fig. 5 includes schematic views showing cross sections of a preferred embodiment of the optical functional sheet according to the present invention, in which a light diffusing phase containing a number of bubbles is used;
Fig. 6 is a view of the apparatus structure showing the relative positioning relationships among respective parts of the backlight; and
Fig. 7 includes diagrams describing the mechanism for achieving the luminance improvement effect of the optical functional sheet according to the present invention.

Reference Numerals

1: a light diffusing phase;
2: a transparent phase;
3: an optical functional sheet;
4: a light diffusive sheet;
5: a light guiding plate;
6: a fluorescent tube.;
7: a reflecting sheet;
8: a phase having s surface of mirror reflecting layer; and
9: a transparent phase having a refractive index different from that of 2.

Best Mode for Carrying Out the Invention

An optical functional sheet of the present invention is characterized in that at least a light diffusing phase and a transparent phase that extend in the direction perpendicular to the surface of the sheet are alternately arranged within the sheet in the direction of the surface of the sheet.
Fig. 1 includes schematic views of a preferred embodiment of the optical functional sheet according to the present invention, showing the vertical cross sections of the sheet. A light diffusing phases 1 and a transparent phases 2 having various cross sectional shapes stand in a relationship in which the central line thereof is normal to the sheet surface, and are alternately arranged in the direction of the surface of the sheet (the lateral direction in the figures). For example, upon mainly considering the cross sectional shape of the light diffusing phase, various types of shapes may be preferably used, such as rectangle (a), square, parallelogram, trapezoid (b), trigon (c), campanulate or bell shape (d), horn shape (e), ellipsoid (f), or other shape such as a modification thereof or the mixture of various shapes. A similar shape to that of light diffusing phase 1 is also preferably used for the cross sectional shape of the transparent phase 2. The present invention includes a case in which respective light diffusing phases 1 and transparent phases 2 extend in the direction generally normal to the sheet surface, as long as the advantageous effect of the present invention is not adversely affected. Here the term "extend in the direction generally normal to the sheet surface" is meant that respective light diffusing phases 1 and transparent phases 2 extend at an angle within +/- 15 degree of the direction normal to the sheet surface.
The light diffusing phase 1 may have a length in the thickness direction of the cross section that is comprised of not less than 50% of total film thickness, and more preferably of not less than 70%. Fig. 1(a) - (f) illustrate examples in which the light diffusing phases 1 and the transparent phases 2 is comprised of 100% in the thickness direction, and Fig. 1(g) and (h) illustrate examples in which the light diffusing phases 1 and the transparent phases 2 is comprised of 70% in the thickness direction, (g) illustrates an example in which a part of the sheet proximate to the upper surface and a part of the sheet proximate to the lower surface are covered by the transparent phases 2, and (h) illustrates an example in which either the upper surface or the lower surface are covered by the transparent phases 2. It is preferable for the transparent phase 2 of the present invention to have a portion that continues from the front surface to the back surface through the direction along the thickness of the film.
The optical functional sheet according to the present invention is characterized in that a light diffusing phase and a transparent phase are alternately arranged within the sheet in the direction of the surface of the sheet. In Figs 1(a) - (h) showing the cross sections, the structure is illustrated, in which the light diffusing phases 1 and the transparent phases 2 are alternately arranged in the direction of the surface of the sheet (the lateral direction in these figures).
The cross sectional shape and the alternately arranged structure of the light diffusing phases 1 and the transparent phases 2 as shown in Fig. 1 may be observed in at least one cross section when the observation is carried out for 18 cross sections that are carved out by every 10 degrees in reference to an arbitrary point on the sheet surface by using the scanning electron microscope or the optical microscope and so on, and preferably carried out on nine patterns and more preferably on 12 or more patterns.
The following three types of the arrangements are preferably used for the surface directional structure of the optical functional sheet according to the present invention. That are, when seeing from the upper side of the sheet surface, (1) an arrangement in which the light diffusing phases 1 are included in the transparent phases 2 in a scattered manner;
(2) an arrangements in which the light diffusing phases 1 coupled in one direction are arranged in the transparent phases 2; and (3) the transparent phases 2 are included in the light diffusing phases 1 in a scattered manner. In each case, either a regular arrangement or a random arrangement may preferably employed for the arrangements. Fig. 2 shows arrangements in the surface direction of the optical functional sheet according to the present invention when seeing in a perspective manner. Fig. 2(a) illustrates an arrangement in which cylindrical light diffusing phases 1 are arranged in the transparent phases 2 (corresponding to (1) described above); Fig. 2(b) illustrates an arrangement in which linear light diffusing phases 1 are arranged in the transparent phases 2 (corresponding to (2) described above); and Fig. 2(c) illustrates. an arrangement in which lattice light diffusing phases 1 are arranged in the transparent phases 2 (corresponding to (3) described above).
Further, in (1), the shape of the light diffusing phases 1 that are disposed in a scattered manner when seeing from the upper side may preferably be circle, ellipsoid, trigon, tetragon, hexagon, modifications thereof or mixtures thereof.
In addition, the optical functional sheet of the present invention preferably comprises the light diffusing phases 1 arranged therein that are mutually coupled to provide a continuous phase, and the transparent phases 2 arranged therein that are decoupled by the light diffusing phases to provide a discontinuous phase.
Here the condition of "the light diffusing phases 1 are mutually coupled" presents a condition of that the surface directional structure of in the sheet surface direction is the state of aforementioned (2) and (3). Fig. 3 and Fig. 4 illustrate the preferable conditions by seeing from the upper side in which the light diffusing phases 1 are mutually coupled.
Fig. 3 illustrates the condition of the aforementioned (2), and more particularly, Fig. 3(a) illustrates a pattern in which the light diffusing phases 1 are linearly extend; Fig. 3(b) illustrates corrugated curves such as sine curve as a typical example; and Fig. 3(c) illustrates a jagged pattern. The pattern is not limited to the patterns disclosed here, and various patterns can be preferably used. In these figures, it is shown that the light diffusing phases 1 are "continuous phases mutually coupled" by observing the elongation direction which is generally linearly continues, and that "the transparent phases 2 are phases decoupled by the light diffusing phases 1" when observing the transverse direction which crosses therewith. Either a regular arrangement or a random arrangement may preferably employed for the arrangements.
Similarly, Fig. 4 illustrates an example of preferable embodiments of the aforementioned (3), which forms a surface configuration in which the transparent phases 2 are included in the light diffusing phases 1 in a scattered manner. The shape of the transparent phases 2 may preferably employ trigon, tetragon, hexagon, circle and ellipsoid and so on. Fig. 4(a) illustrates a case that the transparent phases 2 are circular shaped, Fig. 4(b) illustrates a case for trigonal shaped, Fig. 4(c) illustrates a case for tetragonal shaped, and Fig. 4(d) illustrate a case for hexagonal shaped, and the modifications thereof may preferably be employed, thus it is not limited thereof. Either a regular arrangement or a random arrangement may preferably employ for the transparent phases 2 existing in a scattered manner, regardless of the condition for the arrangement. Most preferable surface configuration for the optical functional sheet according to the present invention is the configuration shown in the aforementioned (3) in which the transparent phases 2 are included in the light diffusing phases 1 in a scattered manner.
Preferable materials for the use of the optical functional sheet according to the present invention are illustrated, which should not be construed as being limited therewith.
Preferable materials employed for the transparent phase 2 are: for example, polyester resins such as polyethylene terephthalate, polyethylene- 2,6-naphthalate, polypropylene terephthalate and polybutylene terephthalate; polyolefin resins such as polyethylene, polypropylene and polymethylpentene; acrylic resins such as poly(metha)acrylate; polycarbonate; polystyrene; polyamide; polyether; polyester amide; polyether ester; polyvinyl chloride; alicyclic polyolefins; and copolymers containing these materials as main components; or transparent resins comprising mixtures of these resins. Transmittance of the transparent phase may be selected so that the collecting or condensing characteristics appeared by the optical functional sheet according to the present invention would not considerably deteriorated, and it is exemplified that haze of the transparent phase having a film thickness of the originally manufactured film is equal to or not higher than 10%.
Next, the light diffusing phase 1 is preferably a transparent matrix component in which fine particulate matter having different refractive index from that of the matrix component is dispersed.
Similar materials to the aforementioned materials for transparent phase can be illustrated for materials for the transparent matrix component of, for example polyester resins such as polyethylene terephthalate, polyethylene -2,6-naphthalate, polypropylene terephthalate and polybutylene terephthalate, polycarbonate, polystyrene, polyolefin resins such as polyethylene, polypropylene and polymethylpentene, polyamide, polyether, polyester amide, polyether ester, polyvinyl chloride, acrylic resins such as poly(metha)acrylate, alicyclic polyolefins, and copolymers containing these materials as main components, or transparent resins comprising mixtures of these resins.
The fine particulate matter dispersed in the transparent matrix component is the diffusion component. Here the materials for the fine particulate matter is not limited to particular quality materials provided that the material has a refractive index different from that of the matrix component, and can illustrate to: for example, crystalline compounds exemplified by spherulites generated from polymer materials; organic compounds exemplified by acrylic resins, organic silicone resins, polystyrene resins, polyurea resins, formaldehyde condensation compounds, fluorocarbon resins, polyolefin resins, polyester resins and so on; inorganic compounds such as glass, silica, barium sulfate, titanium oxide, magnesium sulfate, magnesium carbonate, calcium carbonate and so on; and layers containing gases such as bubbles.
Particle shape of these fine particles is not particularly limited, and particles having various shapes such as spherical form, spheroid form, disk form, rod form, plate form and so on, or infinite form, agglomeration form or so on can be employed. Mean particle size of these fine particulate matter may preferably be 0.1 - 50 µm so as to form sheets having higher transmittance, being achromatically colored and having fine patterns.
The light diffusing phases 1 are created, as the refractive index of the fine particulate matter is different from the refractive index of the transparent matrix. If the refractive index is same between the fine particulate matter and the transparent matrix, refraction followed by scattering does not occur at the interface. In order to obtain the light diffusing phase 1 having substantially effective light diffusivity, the difference in the refractive index between the fine particulate matter and the transparent matrix is not lower than 0.05, and more preferably not lower than 0.1. Smaller refractive index difference of lower than 0.05 presents smaller light diffusing effect.
The transparent matrix that contains a number of bubbles in a dispersed manner to obtain light diffusivity is preferably employed for the light diffusing phase 1 of the optical functional sheet according to the present invention. In such case, it is preferable for the transparent phase 2 to exclude bubbles ("exclude" includes "substantially exclude", which will not cause impairing the effect of the present invention).
Since the refractive index of bubbles, i.e., of air, is as small as 1.0, the refractive index difference can be increase by combining any transparent resin matrix component. For this reason, the efficiency on scattering and reflecting at the interface increases, thereby enabling to produce the light diffusing phase 1 having better diffusivity with thinner thickness.
Mean particle size of bubbles contained in the light diffusing phase 1 is preferably 0.1 - 50 µm, and more preferably 0.1 - 20 µm. Figs. 5(a) - 5(c) are the schematic cross sectional views showing the case in which the light diffusing phases 1 of the optical functional sheet according to the present invention contains a number of bubbles. Circles or ellipsoids shown in these figures schematically represent the shape of bubbles. The shape of bubbles contained in the light diffusing phases 1 my be: spherical form as shown in Fig. 5(a); solenoid form or disk form having elongated axis along a line perpendicular to the surface shown in Fig. 5(b), or spheroid form or disk form having elongated axis along a transverse direction shown in Fig. 5(c), and in addition, modified form thereof, or the same form having their axis along a different direction, or mixed form thereof may also be used. Further, bubbles may be overlapped by any folds either in the thickness direction or the surface direction within the light diffusing phase 1, and the overlap may preferably be equal to or greater than two folds for purpose of obtaining sufficient diffusivity required for the light diffusing phase 1. In addition, porous configuration may also be preferable for the light diffusing phase 1. Here the porous configuration may be employed regardless of the internal structure thereof, as long as the structure internally includes pores.
The optical functional sheet according to the present invention preferably includes the ratio of the length L of the transparent phase 2 in the direction along the film thickness of the sheet to the length p of transparent phase 2 in the direction along the sheet surface (L/p), in any cross section of the sheet, of 2-10.
Fig. 1 shows the length in the sheet thickness direction L of the transparent phase 2. If the light diffusing phases 1 perforated from the front surface to the back surface, the transparent phase length L indicates the film thickness itself (Figs. 1(a) - 1(f)). Also, If the upper or lower portions of the light diffusing phases 1 are covered by the transparent phases as shown in Fig. 1(g) and Fig. 1(h), the light diffusing phase length presents the length L of the transparent phase.
Also, the surface directional length p of the transparent phase 2 is shown in Fig. 1. Fig. 1(a), Fig. 1(g) and Fig. 1(h) illustrate the cases in which the width of the transparent phase is constant along the sheet thickness direction, and the rest of figures of Fig. 1 illustrate the cases in which the width of the transparent phase varies along the sheet thickness direction. Concerning "p" in the case in which the width of the transparent phase varies along the sheet thickness direction, the surface directional length of the transparent phase "p" is selected as shorter one when comparing the length in the upper most portion or in the lower most portion.
The optical functional sheet according to the present invention preferably includes the ratio L/p of 2-10. Concerning an arbitrary cross section for measuring the transparent phase length L and the transparent phase surface directional length p according to the present invention, observations are carried out for 18 cross sections that are carved out by every 10 degrees in reference to an arbitrary point on the sheet surface by using the scanning electron microscope or the optical microscope and so on, and if 2 - 10 of L/p is observed for at least one cross section, the advantageous effect will be appeared.
In addition, smaller advantageous effect can be obtained by having the ratio of out of the above range, and L/p of lower than 2 provides insufficient scattered characteristics presented by the light diffusing phase 1, and L/p of higher than 10 provides excessively higher directivity of the generated beam.
The optical functional sheet according to the present invention preferably includes the ratio of the length L' of the light diffusing phase in the direction along the film thickness of the sheet to the length q of light diffusing phase in the direction along the sheet surface: (L'/q), in any cross section of the sheet, of not less than 1.
Fig. 1 shows the length in the sheet thickness direction L' of the light diffusing phase 1 and the surface directional length q of the light diffusing phase 1. Fig. 1(a), Fig. 1(g) and Fig. 1(h) illustrate the cases in which the width of the light diffusing phase is constant along the sheet thickness direction, and the rest of figures of Fig. 1 illustrate the cases in which the width of the light diffusing phase varies along the sheet thickness direction. Concerning "q" in the case in which the width of the light diffusing phase varies along the sheet thickness direction, the surface directional length of the light diffusing phase "q" is selected as longer one when comparing the length in the upper most portion or in the lower most portion.
The optical functional sheet according to the present invention preferably includes the ratio L'/q of not less than 1. Concerning an arbitrary cross section for measuring the light diffusing phase length L' and the surface directional length q of the light diffusing phase according to the present invention, observations are carried out for 18 cross sections that are carved out by every 10 degrees in reference to an arbitrary point on the sheet surface by using the scanning electron microscope or the optical microscope and so on, and it is preferable to have L'/q of not less than 1 for at least nine cross sections. If more than nine cross sections having L'/q of less than 1 are observed, the ratio of the light diffusing phase 1 dominating within the sheet surface becomes greater, and the masking effect becomes higher and the luminance improvement becomes lower, thus not preferable.
The optical functional sheet according to the present invention preferably includes the areal ratio of the area of the light diffusing phase to the area of the transparent phase within the surface of the sheet of 1/50 - 1/1, and more preferably 1/40 - 1/1.
Having the areal ratio of 1/50 - 1/1 provides being capable of maintaining higher utilization efficiency of the beam transmitted through the optical functional sheet according to the present invention and capable of sufficiently utilizing the scattering characteristics presented by the light diffusing phase 1, thereby providing the improvement on luminance.
Further, in view of processability and of forming the light diffusing phase 1 that can provide sufficient light diffusivity, the film thickness of the optical functional sheet according to the present invention may preferably be 10 µm - 10 mm, and more preferably 10 µm - 5 mm, and, additionally considering the adaptation to the miniaturization on thickness, most preferably 10 -500 µm.
Production methods for the optical functional sheet according to the present invention will be exemplified, though the present invention is not limited thereto.
(1) In the case of the arrangement in which the transparent phase 2 and the light diffusing phases 1 are linearly arranged in the surface direction (Fig. 2(b)), the optical functional sheet according to the present invention is produced by alternately arranging transparent resin sheets and light diffusive sheets via adhesives and slicing them along the direction perpendicularly to the sheet surface. Here, the light diffusive sheet means a sheet containing, for example, diffusive components such as organic fine particles, inorganic fine particles or bubbles in the transparent resins.
(2) Coating film comprising a transparent light-cured resin is formed onto a base sheet. The coating film is optically exposed through a photo mask having a predetermined pattern and then developed, and the unexposed portions are rinsed off to form the pattern. The light diffusing phase 1 is formed by filling the portion rinsed off via the development with a resin paste additionally containing the diffusive components such as fine particles and curing them, and thereby obtaining the optical functional sheet.
(3) Resin additionally containing fine particles or the like is applied across the entire surface of the base sheet, then abrasives are flushed through a mask and the desired shape are chipped off to form the light diffusing phase 1 (sand blasting), and then the portions of chipped off are filled with transparent resins to obtain the optical functional sheet.
(4) The method comprises a step of applying a thermoplastic resin composition containing a photosensitive compound, which is capable of being decomposed by being exposed to light to generate a gas, onto the base sheet, and then exposed with light via a mask.
The photosensitive compound, which is capable of being decomposed by being exposed to light to generate a gas, includes: diazonium salts and their resin compounds such as p-diethylaminobenzenediazonium- zinc chlorate, or -fluoroborate, p-dimethylaminobenzenediazonium- zinc chlorate, or -fluoroborate, 4-morpholino-2,5-dibuthoxybenzenediazonium - zinc chlorate, or -fluoroborate; quinone diazides and their resin compounds such as 1,2- naphtoquinonedizide -5- sodimsulphonate; azide compounds and their resin compounds such as p-azide benzaldehyde, p-azide benzoate, or m- sulfonyl azide benzoate. These photosensitive compounds are blended to resins for matrix component to create the thermoplastic resin composition containing the photosensitive compound which is capable of being decomposed by being exposed to light to generate gas, and the thermoplastic resin is applied onto the base sheet surface to a predetermined thickness. Then, it is covered with a photo mask having a pattern which shields light to portions corresponding to the transparent phases 2, and is exposed to light via the photo mask. The pattern exposure allows the exposed portions to decompose the photosensitive compound contained in the coating film, thereby generating fine gases in the coating film. Subsequently, thermal processing is carried out to soften the thermoplastic resin, and simultaneously make the gases thermally expanded. Accordingly, the light diffusing phases 1 containing a number of gas bubbles in a dispersed manner in the exposed portions are formed, and the unexposed portions become the transparent phases 2 being substantially free of the gas bubbles.
The required optical functional sheet can be obtained by using such process in which air bubbles are mal-distributed within the coating film. Here, for the purpose of improve the thermal stability of the resultant sheet, the matrix component may be cross-linked after forming gas bubbles.
(5) The method comprises the step of employing a material of which the condition is changed between the phase separating condition and the phase solving condition by the thermal actions, and stabilizing the condition. The phase separating condition provides the light diffusing phase 1, and the phase solving condition provides the transparent phase 2. The material may be either: (a) a high temperature phase separation type, having lower critical solution temperature (LCST), exhibiting the phase solution at lower temperature, and inducing the phase separation at higher temperature; or (b) a low temperature phase separation type, having upper critical solution temperature (UCST), exhibiting the phase separation at lower temperature, and exhibiting the phase solution at higher temperature. The phase separating configuration may include: a configuration of being mutually communicated; a configuration of being dispersed with liquid drops; a configuration of mixed thereof or so on. Combination of photo polymerizing materials is preferably employed for stabilizing these conditions.
Flow of the method for forming patterns is as follows. The above indicated pattern forming material is applied onto the surface of the base member, and after the system is solved to become entirely transparent condition by adjusting the thermal conditions, the system is exposed by light via a desired pattern to stabilize the transparency of the exposed portions, thereby forming the transparent phases 2. Then, the unexposed portions are phase-separated to be white haze (light diffusing phases 1) while the transparency of the exposed portions are maintained, and the entire surface of the system is exposed by light as it is, to stabilize the condition of the unexposed portions that have not been exposed during the previous exposure process, thereby obtaining the desired optical functional sheet.
(6) Photo polymerizing composition comprising binder resin, photo polymerizing monomer, photo polymerizing initiator, solvent and so on is applied onto the base member, and the system is subject to a pattern exposure, and the system is immersed into a less soluble solvent to the binder resin, and then the less soluble solvent penetrating the inner portions is dried off by removing via vacuum pressure drying, to form the porous phase. The cured portions via photo polymerization provide transparent phases 2, and the uncured portions are processed to create the porous portions (light diffusing phases 1) to obtain the desired optical functional sheet.
The production methods illustrated above may preferably be accomplished by combining with various other methods.
Also, upon using these methods, it is not limited to use the combination of the transparent phases 2 and the light diffusing phases 1, but may employ the alternate arrangement of the phases having different diffusivities, i.e., having different haze.
When the optical functional sheet according to the present invention having the configuration of the present invention is applied to the backlight of the liquid crystal display, light from the backward can be effectively collected into the normal direction to the sheet surface to provide backlight having higher luminance. The following is a description of the mechanism that enables the luminance improvements.
In Fig. 6, a light diffusive sheet 4 is disposed on upper side surface of a light guiding plate 5, a optical functional sheet 3 is disposed thereon, and a reflective plate 7 is also disposed on under side of the light guiding plate 5. Further, a fluorescent light tube 6 is disposed on the side of the light guiding plate 5. Here Fig. 6 shows the relative spatial relationship between the respective members in an exaggerated manner, and these members mutually contact when used as the backlight. Light generated from the fluorescent light tube 6 enters in the light guiding plate via the side of the light guiding plate 5, and proceeds through the upper surface, the optical functional sheet 4 and the sheet of the present invention 3, and eventually exits toward the upper direction.
The optical functional sheet 3 of the present invention includes the transparent phases 2 and the light diffusing phases 1 that are alternately arranged in a surface direction, and the light diffusing phases 1 act as walls that scatters the light beam.
The aspect of the light beam scattered by the light diffusing phases 1 is shown in Fig. 7.
Light beam entered into the optical functional sheet according to the present invention from the surface (from lower side in Fig. 7) strikes the light diffusing phases 1 and diffusively transmitted or diffusively reflected. It is important for the present invention to suitably arrange the walls (light diffusing phases 1) for diffusively transmitted or diffusively reflected.
The entering light beam originally having the normal direction to the sheet surface(having lower incident angle) is transmitted substantially as it is (Fig. 7(a)), and the light beam entering along the horizontal direction (having higher incident angle) is shielded by the light diffusing phases 1, and is diffusively transmitted or diffusively reflected. Among the light beam diffusively transmitted or diffusively reflected by the light diffusing phases 1, the component having the normal direction to the sheet surface exits thereof, and the component having the direction other than the normal direction to the sheet surface is scattered again by the light diffusing phases 1 (Fig. 7(b)). The iteration of these steps allows the incident light beam to have directivity to the normal direction to the sheet surface, thereby providing luminance improvements for the use for the application of the backlight of the liquid crystal displays.
The important aspect for obtaining the luminance improvement is the surface pattern and the diffusivity of the light diffusing phases 1.
As shown in Fig. 7(c), in the alternate arrangement within the sheet, longer pitches between the light diffusing phases 1 along the surface direction provide insufficient "wall" effect of the light diffusing phases 1 and allow the incident light having horizontal direction passing straight therethrough, thereby providing lower luminance improvement. Also, wider bottoms of the light diffusing phases provide lower light utilization efficiency due to the increase of the probability of being reflected on the bottom of the sheet, thereby decreasing luminance. It is important to design the surface pattern of the light diffusing phases 1 which has suitable pitches while having higher open area ratio.
Further, unlike the light diffusing phases that exhibit diffusive transmission or diffusive reflection of the optical functional sheet according to the present invention, in the case of using phases having mirror surface reflection characteristics (Fig. 7(d)), light beam entering from the underside exits maintaining the incident angle, and therefore the light beam does not collect into the normal direction to the sheet surface, and thus luminance improvement can not be prospected. Also, in the case of the sheet including transparent phases having different refractive index that are alternately arranged (Fig. 7(e)), although some component is refracted toward the normal direction to the sheet surface, total characteristics of the distribution of light including effective light collecting property of normal direction of the sheet surface can not be prospected. This indicates that the diffusive transmission characteristics or the diffusive reflection characteristics of the light diffusing phases 1 are important. In order to achieve improvement on these diffusive transmission characteristics and the diffusively reflecting characteristics, the difference in the refractive index between the diffusive component and the matrix component contained in the light diffusing phases 1 should be set to be larger and the density of the diffusive component is set to be higher (i.e., having more interfaces) or so on.
The optical functional sheet according to the present invention achieves to become sheet having desired light collecting characteristics by adjusting the structure of light diffusing phase.
For one having the surface pattern in which the light diffusing phases 1 are coupled along a single direction as shown in Fig. 3, the light distribution characteristic along the transverse direction is arbitrarily designed by adjusting the array pitches along the transverse direction shown in the figure and so on.
Also, for one having the surface pattern in which the light diffusing phases 1 are coupled along two dimensional direction as shown in Fig. 4, the light distribution characteristic along the two dimensional direction is arbitrarily designed by controlling the surface pattern thereof. For the case of the conventionally used prism sheet, since one ply thereof only provides collecting light in either one direction of transverse or longitudinal, two plies of the prism sheets are necessarily used to superimpose such that respective arrangements of the prisms are arranged in the orthogonal relationship, for the purpose of obtaining the light collecting effect in two dimension - transverse and longitudinal, and the use of the optical functional sheet according to the present invention allows obtaining the light collecting effect in two dimension - transverse and longitudinal with a single ply sheet.
The optical functional sheet according to the present invention is preferably used for the sheet applicable to the backlight of the liquid crystal display, and in such application, improvement of the luminance of the normal direction to the sheet surface can be effectively achieved by piling it onto the light guiding plate or onto the light diffusive sheet.
Since the optical functional sheet according to the present invention exhibits its light collecting characteristics due to the arrangement contained in the interior of the sheet, the optical functional sheet is characterized in that the surface of the sheet can be smooth. For this reason, it is possible to obtain the combined optical functional sheet additionally including other type of optical functional sheet having other functions in a laminated manner without adversely affecting the light collecting characteristics. The combined optical functional sheet allows producing a function integrating sheet exhibiting multiple functions.
Other types of the optical functional sheets for being laminated may include transparent resin sheets and light diffusive sheets. Combining with the transparent resin sheet provides an improvement on the mechanical strength, thermal resistance and handling capability, and combining with the light diffusive sheet provides a function integrating sheet which achieves with one ply thereof the functions that have been achieved with the light diffusive sheet and the prism sheet used in the conventional backlight, thereby simultaneously improving the luminance of the backlight and the luminance uniformity.
The transparent resin sheet available to be used herein includes, for example, polycarbonate resin sheet or the biaxially oriented sheet of polyester resins exemplified as polyethylene terephthalate or polyethylene-2,6-naphthalate.
The light diffusive sheet available to be used herein is preferably a light diffusive sheet incorporating the diffusing function having a configuration in which the transparent matrix component contains fine particles having different refractive index from that of the matrix component in a dispersed manner.
Thickness of other types of the optical functional sheets for being laminated is preferably 20 µm - 500 µm in view of having higher mechanical strength, and more preferably 30 µm - 300 µm, and further preferably 50 µm - 200 µm.
The optical functional sheet according to the present invention can be added with various additives as long as the advantageous effect of the present invention are not deteriorated. Available additives for addition and blending includes, for example, pigments, dyes, optical brightening agents, antioxidants, heat resistant agents, photo resistant agents, antiweatherability agents, antistatic agents, release agents, thickeners, pH adjusters, salts and so on. Also, since the surface is smooth, an antistatic layer or a hard coat layer can additionally be formed thereon.

EVALUATION METHODS

A. Transmittance and reflectance

Transmittance and reflectance were measured by employing spectrophotometer SE-2000 commercially available from NIPPON DENSHOKU INDUSTRIES CO. LTD.

B. Lamination ratio and aspect ratio

A thin tip was carved off from the sheet cross section, and photographs thereof (transfer image) was taken at 400 magnifications by using the optical microscope BH-2 and its camera commercially available from OLYMPUS INDUSTRIES CO. LTD., and the cross section was observed to obtain lamination ratio and aspect ratio thereof.

C. Haze

Transmittance and haze thereof were measured by using automated direct reading haze computer HGM-2DP commercially available from SUGA SHIKENKI Co. Ltd.
Here the term "haze" means a haze obtained by applying the following equations. The haze is a percentage of luminance which is scatter-transmitted by departing from the incident light flux while light from the light source passes through the specimen, and provided that total light beam transmittance is Tt, diffusive transmittance is Td, and direct transmittance is Tp, the total light beam transmittance Tt can be presented by the following equation: Tt = Td + Tp.
Also, haze Ht can be presented as: Ht = 100 x Td/Tt.

EXAMPLES

The present invention will be fully described by referring following examples, though the present invention is not limited thereof.

EXAMPLE 1.

(Sheet 1) A three-layer laminated sheet was manufactured by employing the following formulation:

inner layer polyethylene terephthalate (PET) 98 parts by weight;

polymethylpentene (PMP) 2 part by weight; and

surface layer (on both sides) PET.
These three layers were co-extruded at 280°C, were longitudinally extended to three folds at 85°C, were transversely extended to 3.5 folds at 90°C, and then were thermally processed at 200°C to obtain a sheet having inner layer as diffusion layer of 50 µm and surface layers on both sides as transparent layer of 50 µm. Flat voids were generated around the PMP spherically dispersed in the inner layer. Transmittance was 50%.

(Sheet 2) A three-layered laminated sheet was manufactured by employing the following formulation:

inner layer	copolymer of PET and 17% of isophthalic acid	90 parts by weight
	polypropylene (PP)	10 part by weight; and
surface layer (on both sides)		PET

These three layers were co-extruded at 280°C, were longitudinally extended to three folds at 85°C, were transversely extended to 3.5 folds at 90°C, and then were thermally processed at 230°C to obtain a sheet having inner layer as diffusion layer of 50 µm and surface layers on both sides as transparent layer of 50 µm. Transmittance of the resultant sheet was 90%.

(Sheet 3) Silver was deposited on both sides of a transparent PET sheet having a thickness of 50 µm. The sheet had reflectance of 96%.

The sheets 1 and transparent acrylic sheets having a thickness of 0.4 mm were alternately stuck in piles via adhesive to 30 cm, and the resultant sheet was cut at a thickness of 2.5 mm along the direction perpendicular to the sheet surface. The sheet comprising transparent phases and light diffusing phases arranged in a stripe manner were obtained. The cut sheet was mounted onto backlight with four straight lamps for personal computer monitors, and the luminance of the normal direction to the sheet surface was measured by using luminance meter BM-7 commercially available from TOPCON Co. Ltd. Luminance increased by 25% comparing with the condition of free of the sheet.

EXAMPLE 2

Two sheets prepared in Example 1 containing transparent phases and light diffusing phases that are arranged to form stripes, were stacked in a manner in which the striping arrangements of the sheets are orthogonally aligned, and luminance was measured as in Example 1, and the luminance was increased by 47%.

EXAMPLE 3

Cut sheets were manufactured as in Example 1 except that the sheet 1 was replaced with the sheet 2, and luminance was measured, and the rate of luminance increase was 3%.

COMPARATIVE EXAMPLE 1

Cut sheets were manufactured as in Example 1 except that the sheet 1 was replaced with the sheet 3, and luminance was measured, and the rate of luminance increase was -3%.

EXAMPLE 4

A composition for light diffusing phases having formulation shown below was applied onto a polyethylene terephthalate (PET) sheet to have a dry film thickness of 200 µm:

calcium carbonate(mean particle size 2 µm) 60 parts by weight;

ethyl cellulose 16 parts by weight; and

terpineol 24 parts by weight.
Then a mask for sandblasting having a lattice pattern was mounted thereon, and after that, unwanted portions were cut off. Alumina was employed for abrasives. Subsequently, the cut portions were filled with an ultraviolet cure composition comprising tetrahydrofurfurylmethacrylate, and was exposed to ultraviolet to form transparent phases 2, thereby obtaining desired optical functional sheet. The surface pattern of the resultant optical functional sheet includes square transparent phases 2, as shown in Fig. 2(c), and the pitch between the transparent phase/the light diffusing phase on the surface was 20 µm / 50 µm.
The resultant optical functional sheet was mounted on the backlight with a single straight lamp for notebook sized personal computers in the relative relationship as shown in Fig. 6, and the luminance of the normal direction to the sheet surface was measured by using luminance meter BM-7 commercially available from TOPCON Co. Ltd. Luminance increased by 72% comparing with the condition of free of the sheet. Contrary to the implementation of employing two sheets in the orthogonal manner as in the conventional prism sheet, the present invention provides that required sheet number can be reduced and the required manpower for assembling is reduced.

EXAMPLE 5-7

Optical functional sheets were manufactured as in Example 4 except that the mask patterns for sandblasting were changed to have patterns of the transparent phases in the surface of: perfect circle (Example 5: Fig. 4(a)); regular trigon (Example 6: Fig. 4(b)); and regular hexagon (Example 7: Fig. 4(d)), and light intensities were measured. In each of patterns, the distance between centers of adjacent transparent phases 2 was 70 µm (transparent phase 25 µm/ light diffusing phase 20 µm/ transparent phase 25 µm). The results of the luminance measurements indicate that the rates of luminance increase were: 68% (Example 5); 71% (Example 6); and 72% (Example 7), each of which provides higher luminance improvement.

EXAMPLE 8

(Photosensitive compound)

Following compounds were mixed to form a photosensitive compound:

polyvinyl chloride	150 parts by weight;
polymethyl methacrylate	50 parts by weight;
p-diazo-N,N-dimethyl aniline salt	10 parts by weight;
methyl alcohol	100 parts by weight; and
methylethyl ketone	550 parts by weight.

The photosensitive composition was applied onto a polyethylene terephthalate sheet of 100 µm thick to form a 100 µm thick photosensitive layer. The photosensitive layer was covered with a photo mask having a stripe pattern, and was exposed to ultraviolet of 300 mJ/cm², and after the exposure, a heat processing was carried out at 120°C. Eventually, exposure of the entire surface was carried out with ultraviolet of 300 mJ/cm² to obtain desired optical functional sheet. Generation of air bubbles was confirmed in the exposed portions that were not shielded by the photo mask. The resultant sheet had a pattern having constant pitches along a direction intersectant to the stripe at 20 µm/ 40 µm for the light diffusing phase/ the transparent phase.
The resultant sheet was mounted on the backlight with four straight lamps for personal computer monitors, and the luminance of the normal direction to the sheet surface was measured by using luminance meter BM-7 commercially available from TOPCON Co. Ltd. Luminance increased by 21% comparing with the condition of free of the sheet.

EXAMPLE 9

(Photosensitive compound)

Following compounds were mixed to form a photosensitive compound:

polystyrene	100 parts by weight;
neopentylglycol diacrylate	25 parts by weight;
2-benzyl-2-dimethylamino-1-(4- morpholinophenyl) butanone-1	0.5 part by weight;
cyclohexanone	100 parts by weight; and
tetrahydrofuran	50 parts by weight.

The above-indicated photosensitive composition was applied onto a polyethylene terephthalate sheet of 100 µm thick. The photosensitive layer was covered with a photo mask having a stripe pattern, and was exposed to ultraviolet of 3000 mJ/cm², and was immersed in a methanol bath as it is for 10 minutes. After that, it was dried at a room temperature for 30 minutes. It was confirmed that unexposed portions were porous phases. The resultant sheet had a pattern having constant pitches along a direction intersectant to the stripe at 20 µm/ 100 µm for the light diffusing phase/ the transparent phase. The film thickness was 200 µm.
The resultant sheet was mounted on the backlight with four straight lamps for personal computer monitors, and the luminance of the normal direction to the sheet surface was measured by using luminance meter BM-7 commercially available from TOPCON Co. Ltd. Luminance increased by 15% comparing with the condition of free of the sheet.

EXAMPLE 10

100 parts by weight of polyester resin ("ELITEL" UE3250, commercially available from UNITIKA), 50 parts by weight of acrylic monomer ("BLEMER" AP-150, commercially available from NOF Corporation) and 0.5 part by weight of light polymerization initiator ("IRGACURE" 651, commercially available from Ciba Specialty Chemicals) were dissolved into 100 parts by weight of methylethyl ketone/ cyclohexane mixture (1/1 by weight ratio). The resultant solution was applied onto a polyethylene terephthalate sheet ("LUMIRROR" 100 QT10, commercially available from TORAY) by using a blade coater, then was dried at 80°C for 90 minutes, and after that, was cooled down to obtain a resin sheet of 160 µm.
The sheet was the low temperature phase separation type (UCST) sheet, exhibiting white haze at lower temperature, and exhibiting the phase solution to become transparent by heating at 90°C.
The sheet exhibiting white haze at the room temperature was exposed at 100 mJ/cm² by using ultra high voltage mercury lamp, and the exposed coating film was heated at 100°C, but the white haze remained and not was transformed to transparent phase, and thus the white haze phase was stabilized. The thickness of the coating film was 160 µm, transmittance to light was 76.52%, and haze was 93.54%, thus these conditions presents higher diffusive transmittance.
Further, the resin sheet was heated at 90°C to be a transparent state, and after that, the sheet was exposed at 100 mJ/cm² by using ultra high voltage mercury lamp, while maintaining the transparent state. The resultant sheet was cooled down to the room temperature, and the transparency was not changed to white haze, thus this indicates being successful in stabilizing the compatible phase. The thickness of the coating film was 160 µm, transmittance to light was 91.28%, and haze was 4.60%, thus these conditions presents higher direct transmittance.
Next, the resultant resin sheet was transformed to be the transparent state at 100°C, and thereafter a gap of 100 µm film was sandwiched by disposing thereon a photo mask having a stripe pattern of 50 µm pitches and 30 µm widths, and it was exposed at 35 mJ/cm² by using ultra high voltage mercury lamp. After the exposure, the sheet was cooled down to the room temperature, and after leaving it for one hour, the exposure to the entire sheet was carried out at 1000 mJ/cm² to obtain the stripe pattern.
The resultant coating film was sliced off to obtain thin chips, and the chips were observed by using a optical microscope, and found that an optical pattern was formed having the transparent phase of 30 µm corresponding to the pattern exposed portion and the white haze phase of 20 µm corresponding to the pattern unexposed portion, wherein these phases were alternately arranged and the aspect ratio of the transparent phase 2 was 5.3.

Industrial Applicability

According to the present invention, the optical functional sheet which have a smooth surface that can be surface-treated or be laminated with other functional sheet and which is provided with the light collecting effect due to the internal arrangements thereof, is obtainable. Further, the sheet can be the optical functional sheet involving the internally light collecting function that is capable of collecting both of light diverging transversely and light diverging longitudinally by using a single sheet, thus can be used for applications such as the backlight for the liquid crystal displays. Further, it is capable of obtaining a combined optical functional sheet which includes the light collecting function as well as other functions.

Claims

An optical functional sheet comprising, at least within the sheet, light diffusing phases and transparent phases extending along a direction perpendicular to the surface of the sheet, wherein the light diffusing phases and the transparent phases are alternately arranged along the sheet surface direction.
The optical functional sheet according to claim 1, wherein the light diffusing phases are arranged in a form of continuous phase in which the light diffusing phases are mutually coupled in the direction toward the sheet surface, and the transparent phases are arranged in a form of discontinuous phase in which the transparent phases are decoupled by the light diffusing phases.
The optical functional sheet according to claim 1 or 2, wherein the shape of projective image of the transparent phase onto the sheet surface is selected from the group consisting of trigon, tetragon, hexagon, circle and ellipsoid.
The optical functional sheet according to claim 1 or 2, wherein the light diffusing phase is transparent matrix component, in which fine particulate matter having different refractive index from that of the matrix component is dispersed.
The optical functional sheet according to claim 1 or 2, wherein the light diffusing phase contains a number of bubbles, and the transparent phase does not contain bubbles.
The optical functional sheet according to claim 1 or 2, wherein a ratio of the length L of the transparent phase in the direction along the film thickness of the sheet to the length p of transparent phase in the direction along the sheet surface: (L/p), in any cross section of the sheet, is 2-10.
The optical functional sheet according to claim 1 or 2, wherein the shapes of the cross sections of the light diffusing phase and the transparent phase are selected from the group consisting of rectangle, square, parallelogram, trapezoid, trigon, campanulate or bell shape, horn shape, ellipsoid, or the mixture thereof.
The optical functional sheet according to claim 1 or 2, wherein a ratio of the length L' of the light diffusing phase in the direction along the film thickness of the sheet to the length q of light diffusing phase in the direction along the sheet surface: (L'/q), in any cross section of the sheet, is not less than 1.
The optical functional sheet according to claim 1 or 2, wherein an areal ratio of area of the light diffusing phase to area of the transparent phase within the surface of the sheet is 1/50 - 1/1.
The optical functional sheet according to claim 1 or 2, wherein the film thickness is 10 - 500 µm.
The optical functional sheet according to claim 1 or 2, wherein the sheet is a combined optical functional sheet which is formed by laminating (1) the optical functional sheet described in claim 1 or 2 and (2) other optical functional sheet.
The optical functional sheet according to claim 11, wherein the other optical functional sheet (2) is a light diffusive sheet comprising a transparent matrix component in which fine particulate matter having different refractive index from that of the matrix component is dispersed.
The optical functional sheet according to claim 1 or 2, wherein the sheet is an optical functional sheet adopted for the backlight of the liquid crystal display.